1. Field of the Invention
The present invention relates to a spreading code generation circuit used in a receiver of a spectrum spreading communication and in particular, to a spreading code generation circuit performing phase control by detecting a spreading code generation timing from a received signal which has been subjected to spreading modulation by the direct spreading method upon transmission.
2. Description of the Related Art
In the spectrum spreading communication method used in the mobile body communication or LAN (local area network), when transmitting data, a transmitter performs two-stage modulation to the data: narrow band modulation (primary modulation) and spreading modulation (secondary modulation).
For this, upon reception of data transmitted by radio waves, a receiver firstly performs despreading to the data so as to be returned to the primary modulation state and then a detector circuit reproduces a base band signal. That is, in the receiver, the received data is demodulated in two stages corresponding to the two-stage modulation in the transmitter.
In the aforementioned spectrum spreading communication method, the transmitter performs spreading modulation using a spreading code and the receiver performs despreading using the same spreading code as is used in the spreading modulation.
Special attention is paid on the W-CDMA (wideband code division multiple access) communication method by the spectrum spreading as the mobile body communication method of the next generation. A mobile stations communicating by the W-CDMA communication method starts communication with a base station by performing correlation processing a plurality of types of communication signals transmitted from the base stations, thereby detecting a spreading code to be used in demodulation of the data received from the base station.
Hereinafter, explanation will be given on the spreading code detection method in the spectrum spreading communication method through an example of the W-CDMA communication method.
In the W-CDMA communication method, the spreading code is detected in three stages. When a power is turned on in a mobile station, in order to identify a base station to be communicated, firstly, P-SCH (primary synchronization channel) is caught. The mobile station receives an analog radio signal by an antenna and converts it into a digital signal. By using a spreading code stored in advance, the digital signal is subjected to a despreading processing, so as to detect P-SCH correlation.
The P-SCH is transmitted by one symbol per one slot. The mobile stations detects the analog radio signal synchronization symbol timing by detecting the P-SCH.
Next, as the second stage, the mobile station catches the S-SCH (secondary synchronization channel). When the mobile station receives an analog radio signal by an antenna, the mobile station converts the signal into a digital signal and performs despreading processing to the signal by using a spreading code stored in advance, thereby detecting S-SCH correlation.
The S-SCH is transmitted in parallel to the P-SCH and the mobile station catches the S-SCH at the timing of the synchronization symbol detected at the first stage. Moreover, the S-SCH represents a group of spreading codes used for modulation in the base station and in the second stage, the mobile station can detect the spreading code group.
After the S-SCH is detected at the second stage, the mobile station receives a CPICH (common pilot channel) signal as the third stage and performs despreading of the CPICH signal by using a spreading code belonging to a spreading code group represented by the S-SCH detected.
The CPICH signal has been modulated by using a particular spreading code before transmitted and the mobile station performs despreading of the CPICH signal by a spreading code belonging to the spreading code group. From the correlation result, it is possible to detect the spreading code. The mobile station uses the detected spreading code for demodulation of the data transmitted from the base station to be communicated with.
After completion of detection of the spreading codes, the mobile station generates a spreading code according to the synchronization symbol timing detected and performs despreading at the state synchronized with the received signal, thereby enabling demodulation of a high quality reception data. The receiver used in the mobile station conventionally demodulates received data by using a matched filter as means for detecting synchronization symbol timing and a spreading code generation circuit for generating a spreading code.
Moreover, in the W-CDMA communication method, adjacent base stations can use the same frequency and accordingly, a technique called soft hand over is used. That is, once the adjacent base stations are connected and after complete passing, the original line is cut off. When performing the soft hand over, the mobile station selects and receives a transmission signal from a base station having an intense electric field and frequently switching from one base station to another. For this, the mobile station performs the aforementioned spreading code detection each time the base station is switched.
However, the aforementioned conventional spreading code generation circuit has various problems.
In the receiver of the spectrum spreading communication method, the reception signal should be synchronized with the spreading code in the detection of the spreading code and in demodulation of the reception data. In the conventional spreading code generation circuit, the circuit generating a spreading code (hereinafter, referred to as a spreading code generator) is operated at a high speed so as to advance the spreading code generation timing or operated at low speed or stopped so as to delay the timing, thereby performing phase control so as to obtain synchronization timing of a received signal detected.
The conventional spreading code generation circuit is disclosed in Japanese Patent Publication 7-107006 “Spreading code generation method and apparatus” (Applicant: Sony Co., Ltd.; and inventor: Takehiro Sugita) laid open Apr. 21, 1995. According to this invention, in a receiver of the CDMA communication method, a spreading code generated from the M-series generator is multiplied by a shift vector so as to be shifted by an arbitrary time, which is output to a selector, where a spreading code output with a 1-chip delay is also present. One of them is selectively output from the spreading code generation circuit. In this invention, in order to correspond to the data structure of the spreading code of the CDMA communication method, the operation clock of the M-series generator is operated temporarily at a higher speed than usual or stopped.
Moreover, Japanese Patent Publication 10-173485 discloses “Digital matched filter” (applicant: Mitsubishi Electric Co., Ltd.; Inventor: Kuniyuki Suzuki) laid open on Jun. 26, 1998. According to this invention, a spreading code generated by a spreading code generator circuit is latched in a latch circuit for a predetermined time so as to be delayed and the latch circuit content is updated according to the correlation calculation between the reception data and the spreading code.
However, the conventional spreading code generator circuit has three problems.
The first problem is deterioration of the signal reception characteristic by the phase control on the chip basis. Since the W-CDMA communication is asynchronous communication, synchronization timing of the transmitter and the receiver is shifted due to change of a transmission path environment.
For this, the receiver performs phase control to follow the timing of the transmitter. Conventionally, control has been performed on the chip basis. However, the chip-basis phase control cannot adjust a small timing shift smaller than the chip time, deteriorating the signal reception characteristic.
The second problem is the speed adjustment of the spreading code generation. In the conventional phase control, the spreading code generation circuit has advanced the spreading code generation timing by increasing the frequency of the operation clock of the spreading code generator.
For example, for advancing the generation timing by one frame, i.e., 10 ms, by increasing the clock frequency of the spreading code generator by 4 times, the necessary time is only 2.5 ms. By further increasing the clock frequency, it is possible to advance the phase in a shorter time but the clock frequency that can be set has an upper limit. Moreover, when the clock frequency is increased, power consumption is also increased. Accordingly, for performing a grand phase control, there is a problem that the clock frequency adjustment method is insufficient.
In the spreading code detection processing in the aforementioned W-CDMA communication, a mobile station should detect one spreading code among 512 types. Moreover, the time elapse until detection of the spreading code affects the base station switching time in the soft hand over. Accordingly, reduction of time elapse until synchronization with the reception signal is very important because in the soft hand over an intense signal can be rapidly received, thereby improving the communication quality.
The third problem is a phase control monitoring in multi path. Paths of a transmission signal from the same base station generated in the multi path may increase or decrease in number due to transmission path environment change and the position, i.e., timing may be changed. Moreover, there is a phenomenon that the path position is viewed at a moved position because of the asynchronous communication of the W-CDMA communication method and due to the accuracy of the basic clock of the receiver.
Accordingly, in the receiver, an identified synchronization timing should be slightly shifted for each of the paths to follow it. As a specific method, in the spreading code generation circuit, a phase control amount is monitored for each path so as to perform phase control. Furthermore, in the receiver, a correlation output is performed for each path and RAKE composition is performed as the correlation output result, thereby outputting demodulation result.
Here, the phase control amount is the number of times the phase control (advance/delay) is performed. By monitoring this, it is possible to easily return even when having lost the view of the synchronization timing.
However, conventionally, the phase control amount is performed for each path in the spreading code generation circuit. When the number of paths increases, the entire load on the spreading code generation circuit also increases, affecting the other operation such as spreading code generation and deteriorating demodulation of the reception data.